Total number of neurons — not enlarged prefrontal region — hallmark of human brain

A new scientific study puts the final nail in the coffin of a long-standing theory to explain human's remarkable cognitive abilities: that human evolution involved the selective expansion of the brain's prefrontal cortex.

It does so by determining that the prefrontal region of the brain which orchestrates abstract thinking, complex planning and decision making contains the same proportion of neurons and fills the same relative volume in non-human primates as it does in humans.

"People need to drop the idea that the human brain is exceptional," said Vanderbilt University neuroscientist Suzana Herculano-Houzel, who directed the study. "Our brain is basically a primate brain. Because it is the largest primate brain, it does have one distinctive feature: It has the highest number of cortical neurons of any primate. Humans have 16 billion compared with 9 billion in gorillas and orangutans and six-to-seven billion in chimpanzees. It is remarkable, but it is not exceptional."

In her popular science book The Human Advantage: A New Understanding of How Our Brain Became Remarkable (MIT Press: March 2016), Herculano-Houzel explains how human brains grew so large, even larger than the brains of gorillas and orangutans, whose bodies are larger than ours. Her answer is surprisingly simple. It is the invention of cooking.

Cooking allowed early humans to overcome the energetic barrier that limits the size of the brains of other primates, she has determined. However, when the human brains grew larger they maintained the basic structure of the primate brain, including the size of the prefrontal cortex, her latest study has found. The comparison of the relative size of the prefrontal region in primate brains is described in a paper titled "No relative expansion of the number of prefrontal neurons in primate and human evolution" by Herculano-Houzel and postdoctoral fellow Mariana Gabi published online this week in the Proceedings of the National Academy of Sciences early edition.

The researchers compared the brains of seven non-human primates of varying sizes – pig-tailed and crab-eating macaques, baboon, marmoset, galago, owl monkey and capuchin – with the human brain. They found that both the human and non-human primates devote about 8 percent of their neurons to the prefrontal region of the cortex. In addition, they determined that volumes of human prefrontal gray and white matter match the expected volumes for the number of neurons and other cells in the white matter when compared to other primates.

Cooking allowed us to overcome an energetic barrier that restricts the size of the brains of other primates.""Our big brains are very costly. They use 25 percent of all the energy the body needs each day," Herculano-Houzel said. "Cooking allowed us to overcome an energetic barrier that restricts the size of the brains of other primates."

Take the case of the gorilla. It must spend at least eight hours per day foraging and eating to support its body and brain. The human brain is three times larger than that of the gorilla. If a gorilla had a brain the size of a human, it would have to spend an additional one and a half hours a day finding food. So there simply aren't enough hours in the day for the gorilla to support a bigger brain. Likewise, if humans ate like any other primate, we would have to spend nine and a half hours per day eating – every single day.

That's where cooking comes in. "By cooking, I mean cutting, dicing, smashing-all types of food preparation," Herculano-Houzel said. "Take a single carrot. If you eat it raw, it will take 10 to 15 minutes of vigorous chewing and your digestive system will only capture about one third of the calories. But, if you cut the carrot up and cook it for a few minutes, it takes only a few minutes to consume and your body gets 100 percent of the calories."

The origin of cooking, as Herculano-Houzel defines it, dates back about 2.5 million years ago with the development of the first stone tools. Among other things, these stone tools were man's first food processors, allowing our ancestors to slice and dice and mash their food. Evidence for the controlled use of fire appears about 400,000 years ago.

"Those early tool makers had brains about the same size as gorillas. But, beginning about 1.8 million years ago, the brains of our ancestors began growing steadily, tripling in size over the next 1.5 million years," said Herculano-Houzel.

"It's amazing that something we now take for granted, cooking, was such a transformational technology which gave us the big brains that have made us the only species to study ourselves and to generate knowledge that transcends what was observed firsthand; to tamper with itself, fixing imperfections with the likes of glasses, implants and surgery and thus changing the odds of natural selection; and to modify its environment so extensively (for better and for worse), extending its habitat to improbable locations."

###

Jon Kaas, the Gertrude Conaway Vanderbilt Distinguished Professor of Psychology, and Kleber Neves, Carolinne Masseron, Pedro Ribeiro, Lissa Ventura-Antunes, Laila Torres and Bruno Mota from the Federal University of Rio De Janeiro were co-authors on the paper.

The research was supported by grants from the James S. McDonnell Foundation and the Mathers Foundation.